Various applications require the separation of a light beam, such as laser light, into its constituent wavelength components. For example, many types of lasers provide an output beam having a spectral content concentrated at several discrete wavelengths, referred to as lines. Prism spectrometers may be utilized to separate the laser lines, allowing selected lines to be passed or blocked for various purposes such as laser excitation of samples at a specific wavelength. The laser beam can also be filtered by blocking one or more of the separated laser lines and then recombining the remaining lines into an output beam. In certain optical communication systems it is desirable to be able to separate the several wavelengths in an input beam (demultiplex the beam) to form physically separated beams of different wavelengths, which can then be separately modulated. The modulated beams may be recombined (multiplexed) into an output beam which is transmitted by, for example, a waveguide such as an optical fiber.
Various types of prisms have been used in spectrometers to separate the spectral components of an incoming light beam such as a laser beam. When a multiline laser beam is incident upon a single triangular prism, the individual laser lines exiting from the prism will be both physically separated and angularly dispersed. A second prism can be used to intercept the spreading laser lines and redirect the lines in output beams that are substantially parallel to one another and to the original beam. These physically separated but parallel output beams can then be selectively blocked or passed, or separately modulated, depending on the preferred application. However, the use of two (or more) prisms in this manner results in the output beams all being displaced from the input beam. In addition, to obtain sufficient physical separation of the laser lines that exit after passing through the two prisms, it may be necessary to orient the prisms so that the beams strike the prisms at angles substantially different than the Brewster's angle for the wavelength components of the beam, resulting in excessive loss of beam intensity.